Method for improving the control performance of a motor vehicle control system

ABSTRACT

To improve the control behavior of an automotive vehicle control system, such as an anti-lock system (ABS), a driving stability control system (ESP, ASMS, DDC), etc., in a brake system which includes a master cylinder, inlet and outlet valves for pressure modulation, a low-pressure accumulator, and a hydraulic pump for returning the pressure fluid discharged, when a wheel becomes unstable and upon commencement of the ABS control, that branch of the control system to which the unstable wheel is connected is temporarily uncoupled from the master cylinder pressure (p HZ ) or initial pressure by closing the pressure fluid conduit in the direction from the wheel brakes to-the master cylinder, and a relatively quick rise of the braking pressure and a quick approach of the wheel braking pressure to the master cylinder pressure (p HZ ) and, thus, the wheel lock pressure level is caused by returning pressure fluid from the low-pressure accumulator into this branch. A special valve which can provide a connection between the master cylinder and the suction side of the hydraulic pump is switched over to open-passage-only after the low-pressure accumulator has been emptied at least in part, or as required.

TECHNICAL FIELD

The present invention generally relates to a method for improving thecontrol characteristics of automobiles and more particularly relates tousing the vehicle's hydraulic brake system with a closed-loop controlsystem for improving vehicle stability

BACKGROUND OF THE INVENTION

It can be observed in automotive vehicle control systems known in theart and used at present for anti-lock control (ABS) or driving stabilitycontrol (ESP, ASMS, DDC) that with an initial pressure (pressure in themaster cylinder) which is only slightly in excess of the wheel lockpressure level, a relatively poor (i.e. below the theoreticallyattainable value) brake performance is achieved because the controlalgorithms and control operations which aim at an optimum braking effectrequire an optimal, i.e. high initial pressure, as a precondition, orthey are dimensioned for high initial pressures. To take into accountand detect situations with low initial pressures which lie only slightlyabove the wheel lock pressure level, it is known in the art to implementdefined learning algorithms which accordingly modify pressure modulationin response to relatively low initial pressures. However, a certainperiod of time lapses until these learning algorithms are optimallybuilt up, with the result that precious braking effect is wasted and thestopping distance becomes longer than necessary.

In WO-A-97 13670 a method of operating a hydraulic brake system isdescribed which assumes a brake system that includes brake slip control,traction slip control, and yaw torque control. The brake system shall beappropriate for active braking. The term “active braking” exclusivelyrefers to a traction slip control operation or a yaw torque controloperation by brake intervention, and brake intervention occurs withoutthe driver's intent, i.e. without application of the brake pedal.

WO-A-96 02409 discloses a method of operating a wheel-lock controlledautomotive vehicle brake system for driving stability and/or tractionslip control operations. The objective of this method is to ensureprefilling of the wheel brakes with pressure fluid in order that thewheel brakes can be acted upon with braking pressure according todriving stability and/or traction slip control, although the driverhimself/herself does not apply the brake pedal.

Finally, German patent application No. 39 35 595 describes a brakesystem for controlling a wheel brake of a vehicle wherein a separatemagnetically-operated two-position/three-way valve in each circuit ofthe brake system is used to control pressurization of the respectivebrake circuit. The application further discloses a method-of preventingan imminent locked condition of the rear wheels.

Experience shows that the ‘normal driver’ applies the brake onlyhesitantly in many situations so that an initial pressure which issufficient for an optimal controlled braking operation is not achievedat all, or at a too slow rate.

An object of the present invention is to eliminate the difficultieswhich occur especially at relatively low initial pressures, i.e. initialpressures which lie only slightly above the wheel lock pressure level,and to improve the control operation in such situations by enhancedutilization of the prevailing coefficient of friction or road surfaceconditions, thereby increasing the braking effect.

The method of the present invention achieves the objectives byimplementing the following method. Upon commencement of the control,that branch of the braking pressure control system to which the wheeldetected as unstable is connected is temporarily uncoupled from themaster cylinder pressure or initial pressure by closing the pressurefluid conduit in the direction from the wheel brake to the mastercylinder of the brake system, and that by returning pressure fluid fromthe accumulator (low-pressure accumulator) into the branch to which theunstable wheel is connected, a relatively quick rise of the brakingpressure in the control branch or, respectively, a quick approach of thewheel braking pressure to the wheel lock pressure level is caused(‘quick’ meaning: compared to the braking pressure rise withoutuncoupling).

Even at a relatively low initial pressure, the method of the presentinvention permits achieving a pressure increase and an approach of thepressure in the mentioned branch and, thus, also in the wheel thatbecame unstable to the wheel lock pressure level, which is relativelyquick in comparison to conventional systems.

Upon commencement of the control, the separating valve is closed and thepressure fluid required for pressure build-up is initially taken fromthe accumulator (low-pressure accumulator), whereupon the special valvewill be opened, i.e. switched to open passage, only after theaccumulator has been emptied at least in part.

It has proved especially suitable to measure the degree of filling ofthe accumulator or the quantity of pressure fluid stored in theaccumulator or to determine them in approximation by producing aso-called volume model, and to control the opening of the special valvein dependence on the degree of filling or the pressure fluid quantity.It is important that the fluid volume required for the pressure build-upin the wheel brakes is at disposal on the suction side of the hydraulicpump when it is needed. This fluid volume is either taken from theaccumulator or supplied from the master cylinder by way of the specialvalve. Thus, for example, the special valve will only be opened orswitched over to open passage as soon as the degree of filling or thevolume in the accumulator has fallen below a predetermined minimumvalue.

In another embodiment of the present invention, the current pressurewhich is instantaneously prevailing in the master cylinder, is comparedto the current deceleration of the vehicle or a pressure value whichcorresponds to. the deceleration, respectively. As is known, there is adefined relation between the pressure and the vehicle deceleration whichcan be achieved due to this braking pressure. When the comparisonbetween pressure and deceleration shows that the vehicle decelerationexceeds a plausible value which corresponds to the current pressure inthe master cylinder, this indicates that the vehicle brakes at a higherrate than the driver desires. The special control of the presentinvention ‘oversteers’, or there is an error. Therefore, switch-back toa standard braking pressure control is effected where the pressureupstream of the inlet valves to the wheel brakes is coupled to theinitial pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the most important hydraulic components of a brakesystem in which the method of the present invention is implemented.

FIG. 2 shows in a flow chart the variation of some control quantities(wheel pressure, initial pressure, vehicle reference speed, and wheelspeed) during a braking operation in which the method of the presentinvention is used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment of FIG. 1 shows a hydraulic brake system which isappropriate for anti-lock control (ABS) and traction slip control (TSC)as well as for driving stability control (ESP, ASMS, DDC). It will onlybe explained hereinbelow what is required to implement the method of thepresent invention.

The system shown in FIG. 1 is a dual-circuit hydraulic system with aclosed-loop hydraulic system or braking pressure control system,respectively. The wheel brakes are connected to a master cylinder 1(tandem master cylinder) by way of two brake circuits I, II of identicaldesign. Therefore, it is sufficient to describe one of the two brakecircuits, i.e. brake circuit I, in the following.

The components illustrated upstream of the master cylinder 1 are usedfor precharging. These components have no significance for the method ofthe present invention and, therefore, are not explained in detail.

Connected to the brake circuit I are wheel brakes 3 and 4 of the twofront wheels FR, FL by way of one pair of inlet/outlet valves 5, 6; 7,8, respectively. These valves are electromagnetically operabletwo-way/two-position directional control valves. The outlet valves 6, 8lead to a low-pressure accumulator 9 which is evacuated by way of ahydraulic pump 10, the electric drive motor of which is not shown. Anumber of non-return valves are still inserted into the evacuationconduit for known reasons and in a conventional manner.

A separating valve 11 (or a so-called TSC-valve) is inserted in eachbrake circuit I (of course, also in II, which is not described though)in the pressure fluid conduit from the master cylinder 1 to the wheelbrakes. Valve 11 is a two-way/two-position directional control valvewhich is open in its (denergised) inactive position and is bypassed by anon-return valve 12 in the direction of pressure fluid flow from themaster cylinder 1 to the wheel brakes 3, 4 and the inlet valves 5, 7,respectively. Further, a pressure-limiting valve 13 is connected inparallel to the non-return valve 12. Separating valve 11 is requiredespecially in a traction slip control (TSC) operation because brakingpressure for intervention into the wheel brakes is required in suchsituations when the braking pressure generator is not actuated andmaster cylinder 1 is unpressurised.

Further, the brake system according to FIG. 1 includes a special valve14, i.e., a two-way/two-position directional control valve which isclosed in its inactive position and can be switched over electrically toopen passage. In case of requirement, pressure fluid can be conducted byway of this special valve from the master cylinder 1 to the suction sideof the hydraulic pump 10.

Finally, the brake system according to FIG. 1 is still equipped withpressure sensors 15 which permit measuring the actual initial pressureor, respectively, the hydraulic pressure built up in the two circuits I,II of the master cylinder 1.

Further comprised in a brake system of the type shown in FIG. 1 arewheel sensors 16, 17 by which the rotational behavior of the individualwheels can be determined and evaluated for braking pressure control.

The electronic unit for conditioning and evaluating the sensor signalsand for generating the pump actuating signals and the valve actuatingsignals is not illustrated in FIG. 1 for the sake of clarity. Generally,electronic controllers with programmed circuits (microcomputers,microcontrollers, etc.) are nowadays used which permit computing thebraking pressure control signals from the wheel sensor signals and,possibly, from other input quantities on the basis of complexalgorithms, and producing the signals required to control the pump andactuate the valves.

The embodiment of FIG. 2 serves to describe the operation and mode ofeffect of the method of the present invention. To explain the actions inthe course of the method of this invention, FIG. 2 shows the pressurevariation in the master cylinder p_(HZ) (dotted lines), the pressurevariation p_(Rad) in the wheel brake of the wheel observed in the normalcase or a standard braking pressure control operation, the brakingpressure variation p_(SR) in this wheel when the special control of thepresent invention is employed, and further the speed variation v_(Rad)of the wheel which became unstable at the time observed, and finally thevariation of the vehicle speed or vehicle reference speed v_(Ref). Thesame time scale t applies in all curves or procedures. The brakingoperation observed herein commences at the time t₀, and instabilitystarts at t₁.

A critical braking situation in which the present invention isimplemented prevails when in a braking operation with a relatively lowinitial pressure p_(HZ) a wheel becomes unstable and anti-lock controlsets in. t, marks the commencement of the control. The braking pressurep_(Rad) is reduced so that the wheel recovers, becomes stable again(t₂), and braking pressure can be built up again. As a result of therelatively low initial pressure p_(HZ), the braking pressure followingt₂ will rise only relatively slowly in such a case and approach thewheel lock pressure or initial pressure at a relatively slow rate. Thepressure variation in a standard control operation is referred to asp_(SD) in FIG. 1. Of course, stopping distance is “wasted” due to thisslow braking pressure build-up.

In order to completely use the existing coefficient of friction, a muchsteeper pressure rise compared to the pressure variation p_(SD) and amuch quicker approach to the wheel lock pressure level or to the initialpressure p_(HZ) is desired. When the method of the present invention isemployed, that is the special control of the present invention, thebraking pressure variation referred to by p_(SR) and, thus, aconsiderable increase of the braking effect is achieved. According tothe present invention, this is achieved by “uncoupling” the brakingpressure in branch 18 of the braking pressure control system, i.e., thepressure at the inlet of the inlet valves 5, 7, at the commencement ofan anti-lock control operation, i.e., when the control commences at timet₁ (FIG. 1). The pressure fluid which is conducted by way of the outletvalves 6, 8 into the low-pressure accumulator 9 in the phase of pressurereduction following t₁ is supplied from the low-pressure accumulator 9by way of the pump 10 into the branch 18 in order to develop a highsystem pressure in branch 18 and, thus, to quickly increase the brakingpressure in wheel brakes 3, 4. This uncoupling action is carried out byswitch-over of the separating valve 11 into the closed condition. Whenthe initial pressure in the master cylinder 1 increases, pressure fluidis not impeded to propagate into branch 18 by way of the non-returnvalve 12.

The special valve 14 is closed at the commencement of the control sothat the low-pressure accumulator 9 is initially emptied. As soon as theamount of volume removed or the degree of filling of the accumulator 9falls below a limit value, the special valve 14 is switched to openpassage so that now pressure fluid can propagate from the mastercylinder 1 to the suction side of the pump 10. Change-over of thespecial valve 14 into the open condition can be controlled in dependenceon different criteria, such as, the still existing residual quantity inthe low-pressure accumulator 9, in dependence on opening of the inletvalve 5, 7, etc.

Summarizing, the operation and mode of effect of the circuit of FIG. 1which is equipped with pressure sensors 15 on the master cylinder 1 canbe described as follows.

When commencement into ABS control is detected at one or more wheels,the separating valves or TCS-valves 11 are closed. Further pressureincrease initiated by the driver is possible any time by way of theparallel connected non-return valves 12. Initially, the special valves14 remain closed. The pressure fluid volume discharged during pressurereduction following the detection of instability is aspirated by thepump 10 via the low-pressure accumulators 9 and serves for the systempressure increase upstream of the inlet valves 5, 7. In the absence of apressure reduction phase in the corresponding brake circuit, or if nofurther pressure reduction is expected, the fluid volume supply of thepump 10 is ensured by opening of the separating valves 14 so that nowthe pump 10 is delivered with fluid directly from the master cylinder 1,with the result that system pressure development (in branch 18) ispermitted. Knowing about the degree of filling of the low-pressureaccumulators 9 permits a delay in opening the special valves 14 untilthe complete evacuation or critical evacuation of the low-pressureaccumulators 9. It is also possible to open the special valves 14synchronously to the inlet valves 5, 7.

The degree of filling in the low-pressure accumulators 9 can be measuredby a corresponding sensor. However, for cost reasons, it is preferred todetermine the degree of filling by a volume model or low-pressureaccumulator model, reproduced by software, which is derived e.g. fromvalve actuation operations in a per se known manner.

The system pressure in the brake circuit is limited by a pressure-reliefvalve 13.

Detection of the driver's intent is carried out by means of pressuresensor 15 in the present embodiment. The pressure measured in the mastercylinder 1 is associated with a pressure/deceleration characteristiccurve of the vehicle that is stored in the software in the controller(not shown). This value is compared with the deceleration computed fromthe wheel sensor signals. In case the actual vehicle deceleration isconsiderably in excess of the deceleration which corresponds to theinitial pressure, i.e., the pressure in the master cylinder 1, thisindicates that the driver wishes a reduced deceleration. In this case,the special control of the present invention is terminated, andswitch-over to a standard braking pressure control is made in order toprevent excessive braking which goes beyond the brake command issued bypedal application.

It is principally also possible to employ the method of the presentinvention in a brake system which is equipped with a so-called brakeassist function. In this case, the system pressure increase is caused byactuation of the brake assist system when commencement of ABS control isdetected at one or more of the wheels.

When the driver wishes to reduce the deceleration, the release switchimplemented in the brake assist system will be actuated and the specialcontrol according to the present invention thereby terminated. Again,the standard braking pressure control will set in.

Forced braking on a road surface with abrupt changes in the coefficientof friction is expediently prevented by a long-time monitoring of thewheel rotational behavior (all wheels stable).

The method of the present invention proves extremely favorable. Becausean “optimal initial pressure” is always available to the system, longpressure increase series are avoided which would cause a non-homogeneousdeceleration and losses in braking effect.

With the method of the present invention, the “normal driver” ispermitted an optimal utilization of the coefficient of friction even ifthere are certain operating errors, in particular, a too weak brakepedal application.

Controlled braking operations at low initial pressures frequently causestability problems in braking operations during cornering when thestandard control algorithms are applied. These difficulties, too, arelessened or even eliminated by the method of the present invention.

Increasing the braking pressure is possible to the driver in everysituation by increase of the initial pressure, which offers a highdegree of safety and prevents braking at too low forces.

What is claimed is:
 1. Method of controlling an anti-lock system (ABS)of a vehicle, the system including a master cylinder, inlet and outletvalves, a separating valve and a special valve interposed between themaster cylinder and the inlet valves, a low-pressure accumulator, and ahydraulic pump, the method comprising the steps of: switching from astandard braking pressure control mode to a special braking pressurecontrol mode, upon commencement of the ABS, by uncoupling from themaster cylinder that branch of the control system to which a wheeldetected as unstable is connected by closing the separating and specialvalves in that branch of the control system to cause fluid to propagatefrom the low-pressure accumulator into that branch of the control systemto which the wheel detected as unstable is connected, and opening thespecial valve when a level in the low-pressure accumulator falls below apredetermined level such that fluid propagates from the master cylinderto the suction side of the hydraulic pump, thereby rapidly increasing apressure of the fluid in that branch of the control system to which thewheel detected is unstable.
 2. Method according to claim 1, whereby anactuation of an inlet valve in that branch of the control system towhich the wheel detected as unstable is connected occurs when thespecial valve is opened.
 3. Method according to claim 1, furtherincluding the step of determining a fluid pressure in the mastercylinder by means of a characteristic curve stored in a controller. 4.Method according to claim 3, further including the step of comparing thecharacteristic curve to a deceleration of the vehicle.
 5. Methodaccording to claim 4, further including the step of switching to astandard braking pressure control without uncoupling that branch of thecontrol system to which a wheel detected as unstable is connected whenthe deceleration of the vehicle exceeds a value corresponding to thefluid pressure in the master cylinder.